This invention relates to a titania-zirconia or titania-zirconia-alumina powder and a process for producing the same. More particularly, it relates to a novel titania-zirconia or titania-zirconia-alumina powder which is suitable as a catalyst carrier or a co-catalyst, particularly for purifying automotive exhaust gases or as a catalyst carrier for use in high temperature gases containing sulfur, and a process for producing the powder.
In order to improve the properties of titania powder used as a catalyst carrier or a co-catalyst, zirconia is frequently used as an additive in combination with an alkaline earth metal, a transition element, and a rare earth element.
For example, JP-A-58-143839 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a catalyst for purifying nitrogen oxides comprising (A) a modified complex oxide obtained by treating a titanium-zirconium binary complex oxide with barium, (B) a vanadium oxide, and (C) an oxide and/or a sulfate of at least one element selected from the group consisting of tungsten, molybdenum, tin, chromium, manganese, cerium, and iron. Thus, it is known that the combined use of zirconia provides a catalyst which exhibits high activity in a broad temperature range and at a high space velocity without being influenced by oxygen, SOx, carbonic acid gas, steam, halogen compounds, and hydrocarbons present in exhaust gases or by smog and has a small ability of oxidizing SO2 to SO3.
JP-A-8-192051 discloses a catalyst for purifying exhaust gases which comprises a carrier comprising a titanium-zirconium complex oxide, an NOx storage component selected from among alkali metals, alkaline earth metals and rare earth elements and supported on the carrier, and a noble metal supported on the carrier. Accordingly, it is known in the art that a catalyst having a titanium-zirconium complex oxide as a carrier is less susceptible to poisoning by sulfate or sulfite ions than a catalyst having an alumina carrier, that a sulfate of an NOx storage component generated by the adsorbed sulfate or sulfite ions easily decomposes at a low temperature, and that the titanium-zirconium complex oxide carrier has improved heat resistance and improved acidity compared with a titanium carrier.
In this way numeral attempts have been made to date to impart heat resistance to titania and to improve the surface acidity of titania by addition of zirconia while retaining the resistance of titania against sulfur poisoning. However, it has been unknown that the contemplated effects can be enhanced by solid dissolving zirconia in an anatase phase of titania or by solid dissolving titania in a tetragonal phase of zirconia and that use of such a solid solution powder as a carrier provides a catalyst with markedly increased performance.
JP-B-7-24774 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) proposes an oxidizable substance- and/or nitrogen oxide-containing carrier for a catalyst for treating exhaust gases, which comprise an inorganic refractory oxide obtained by heat-treating a material containing a titanium compound and a zirconium compound at 660 to 900xc2x0 C., the inorganic refractory oxide comprising 20 to 90 mol % of TiO2 and 10 to 80 mol % of ZrO2 and containing at least 20% by weight of a titanium-zirconium complex oxide having a crystal structure of ZrTiO4. This proposal contemplates providing a carrier which provides a catalyst having high purifying performance and improved heat resistance by taking advantage of the strong solid acidity and higher heat resistance of ZrTiO4 than the heat resistance of titania alone or zirconia alone. It is essential, therefore, for the inorganic refractory oxide to contain at least 20% by weight of ZrTiO4.
However, JP-B-7-24774 does not mention the importance of preventing sulfur poisoning and of high-temperature non-reactivity with an alkaline salt, which is an NOx storage component, in NOx-storage and reduction type catalysts; still less suggests that it is rather a key point for this importance that titania-zirconia powder be free from basic sites on its surface.
It is utterly unknown that high performance could be exerted in terms of the above-mentioned sulfur poisoning resistance and non-reactivity with an NOx storage component, not in an area mainly comprising ZrTiO4, but in a titania-rich solid solution system mainly comprising an anatase phase or in a zirconia-rich solid solution mainly comprising an tetragonal phase.
JP-A-6-304477 describes an amorphous zirconium-titanium complex oxide (partly crystalline but totally amorphous) having a ZrO2: TiO2 weight ratio of 5:95 to 95:5 and a process for producing the same. However, as is taught in JP-B-7-24774 supra, a zirconium-titanium complex oxide obtained by the general co-precipitation method comes to have a composition mainly comprising ZrTiO4 on being heat-treated at 700xc2x0 C. or higher temperatures. The crystal structure according to the present invention (a structure having titania solid-dissolved in a tetragonal phase of zirconia, with a small amount of ZrTiO4 or a monoclinic phase of zirconia being present around the solid solution grains; hereinafter described in detail) is not obtained by the co-precipitation method. Further, JP-A-6-304477 merely describes the amorphous complex oxide of zirconium and titanium, only reciting the general co-precipitation method as a method of production. Accordingly, it is apparently recognized that the amorphous (partially crystalline but totally amorphous) zirconium-titanium complex oxide disclosed in JP-A-6-304477 would be inferior in heat resistance and alkali resistance when used as a carrier for a NOx-storage reduction type catalyst, as is with the oxidizable substance comprising the inorganic refractory oxide disclosed in JP-B-7-24774. It is absolutely unknown that it is important for titania-zirconia powder to be not only amorphous but be capable of maintaining a large specific surface area even after heat treatment.
We find no literature on attempts to improve heat resistance of titania powder without changing its characteristic that there are only acidic sites with few basic sites on the surface. It is also unknown that a titanium-zirconium oxide solid solution having zirconia solid-dissolved in an anatase phase of titania or having titania solid-dissolved in a tetragonal phase of zirconia exhibits both excellent heat resistance and sulfur poisoning resistance.
With regard to an oxide solid solution, JP-A-9-221304 discloses a cerium-zirconium oxide solid solution but has no mention of an oxide solid solution having zirconia or titania solid-dissolved in titania or zirconia, respectively.
Commercially available titania powder (titania powder having an anatase crystalline phase) is so inferior in heat resistance that it extremely reduces the specific surface area on heating at high temperature. That is, it is unable to maintain a desired specific surface area after completion of high-temperature treatment. None of available titania powders, except of TiO2xe2x80x94SiO2 system, maintains a specific surface area of 45 m2/g or more when heated at 800xc2x0 C. for 5 hours in the air.
JP-A-9-926 discloses a catalyst for purifying exhaust gases which comprises a carrier of a Tixe2x80x94Zrxe2x80x94Al complex oxide having supported thereon an NOx storage component and a noble metal catalyst. It is hence known as disclosed that a combination of titania, zirconia, and alumina enjoys both the effects of the Tixe2x80x94Zr complex oxide in preventing sulfur poisoning and improving heat resistance and the effects of an aluminum oxide in further improving the heat resistance. However, it is not known in the art that these effects produced by the Tixe2x80x94Zrxe2x80x94Al ternary oxide in prevention of asulfur poisoning and heat resistance improvement can be enhanced where the Tixe2x80x94Zr complex oxide in this ternary oxide system is an oxide solid solution having zirconia or titania solid-dissolved in titania or zirconia, respectively.
The social recognition of the importance of environmental conservation has ever been growing, and the demand for purifying exhaust gases from automobiles, etc. has been increasing. In this situation, the purifying performance of the background art catalysts is far below the desired levels.
An object of the present invention is to provide a novel titania-zirconia powder and a novel titania-zirconia-alumina powder that have high resistance against poisoning with acidic substances, such as sulfur compounds, easily desorb such poisoning substances having been once adsorbed, and are capable of maintaining a high specific surface area even on being heated in a high temperature.
Another object of the invention is to provide a novel titania-zirconia powder and a novel titania-zirconia-alumina powder which exhibit markedly improved heat resistance without impairing the characteristic of titania""s having few basic sites on the surface.
A further object of the invention is to provide processes for producing these novel powders.
The objects of the present invention are accomplished by the following embodiments:
(1) A titania-zirconia powder, wherein at least a part of the zirconia is dissolved in the crystalline phase of titania to form a solid solution, or at least a part of titania is dissolved in the crystalline phase of zirconia to form a solid solution.
(2) An amorphous titania-zirconia powder, being a titania-zirconia powder wherein at least apart of the zirconia is dissolved in the crystalline phase of titania to form a solid solution, or at least a part of titania is dissolved in the crystalline phase of zirconia to form a solid solution after heat treatment.
(3) The titania-zirconia powder as set forth in (1), comprising a zirconia in an amount of 3 to 30% by weight, and having a specific surface area of 45 m2/g or more after being heat-treated at 800xc2x0 C. for 5 hours in the air, wherein the titania-zirconia powder comprises an anatase phase, and at least one of a complex oxide having a composition of ZrTiO4 or (Ti,Zr) O2, monoclinic phase zirconia, and tetragonal phase zirconia is contained in an amount of less than 20% by weight in total.
(4) The titania-zirconia powder as set forth in (3), further comprising an yttria in an amount of 0.5 to 10% by weight, and having a specific surface area of 34 m2/g or more after being heat-treated at 900xc2x0 C. for 5 hours in the air.
(5) The titania-zirconia powder as set forth in (3), wherein at least a part of the zirconia is dissolved in the anatase phase to form a solid solution, and the at least one of the complex oxide, monoclinic phase zirconia, and tetragonal phase zirconia is dispersed in the anatase phase.
(6) The titania-zirconia powder as set forth in (4), wherein at least a part of the zirconia is dissolved in the anatase phase to form a solid solution, and wherein at least one of the complex oxide and the tetragonal phase zirconia is dispersed in the anatase phase.
(7) The amorphous titania-zirconia powder as set forth in (2), being amorphous in the proportion of 90% by volume or more, comprising 60 to 85% by weight of zirconia, and having a specific surface area of 140 m2/g or more after being heat-treated at 500xc2x0 C. for 5 hours in the air.
(8) The titania-zirconia powder as set forth in (1), comprising a titania in an amount of 15 to 40% by weight, and having a specific surface area of 35 m2/g or more after being heat-treated at 800xc2x0 C. for 5 hours in the air, wherein the titania-zirconia powder comprises a tetragonal crystalline phase, at least a part of the titania is dissolved in said tetragonal phase to form a solid solution, and at least one of a complex oxide having a composition of ZrTiO4 or (Ti,Zr)O2, monoclinic phase zirconia, and anatase phase titania is contained in an amount of less than 20% by weight in total.
(9) The amorphous titania-zirconia powder as set forth in (7), being a titania-zirconia powder having a specific surface area of 35 m2/g or more after being heat-treated at 800xc2x0 C. for 5 hours in the air, wherein the titania-zirconia powder comprises a tetragonal crystalline phase, at least a part of the titania is dissolved in the tetragonal phase to form a solid solution, and at least one of a complex oxide having a composition of ZrTiO2 or (Ti,Zr)O2, monoclinic phase zirconia, and anatase phase titania is contained in an amount of less than 20% by volume in total.
(10) A catalyst carrier or a co-catalyst for purifying automotive exhaust gases which comprises the titania-zirconia powder of (1).
(11) A catalyst carrier or a co-catalyst for purifying automotive exhaust gases, comprising the amorphous titania-zirconia powder of (2).
(12) The titania-zirconia powder as set forth in (3), having an average particle size of 1 xcexcm or smaller, and dispersed in an alumina powder.
(13) The titania-zirconia powder as set forth in (7), having an average particle size of 1 xcexcm or smaller, and dispersed in an alumina powder.
(14) The amorphous titania-zirconia powder as set forth in (8), having an average particle size of 1 xcexcm or smaller, and dispersed in an alumina powder.
(15) The titania-zirconia powder as set forth in (12), wherein the alumina powder has an average particle size of 1 xcexcm or smaller.
(16) The titania-zirconia powder as set forth in (13), wherein the alumina powder has an average particle size of 1 xcexcm or smaller.
(17) The amorphous titania-zirconia powder as set forth in (14), wherein the alumina powder has an average particle size of 1 xcexcm or smaller.
(18) The titania-zirconia powder as set forth in (3), having an average particle size of 0.1 xcexcm or smaller and uniformly dispersed in an alumina powder having an average particle size of 0.1 xcexcm or smaller.
(19) The titania-zirconia powder as set forth in (7), having an average particle size of 0.1 xcexcm or smaller and uniformly dispersed in an alumina powder having an average particle size of 0.1 xcexcm or smaller.
(20) The titania-zirconia powder as set forth in (8), having an average particle size of 0.1 xcexcm or smaller and uniformly dispersed in an alumina powder having an average particle size of 0.1 xcexcm or smaller.
(21) A process for producing a titania-zirconia powder comprising the steps of:
(a) preparing a raw material aqueous solution comprising a titanium salt and a zirconium salt, the concentrations of the titanium salt and the zirconium salt being adjusted so as to give a final titania,/zirconia weight ratio of 97/3 to 70/30 or 40/60 to 15/85,
(b) adding a pH adjusting agent to the raw material aqueous solution while powerfully stirring to form a precipitate, and
(c) drying and calcining the precipitate.
(22) The process for producing a titania-zirconia powder as set forth in (21), wherein the raw material aqueous solution in step (a) further comprises aqueous hydrogen peroxide and a surface active agent.
(23) The process for producing a titania-zirconia powder as set forth in (21), wherein the raw material aqueous solution in step (a) further comprises yttrium in an amount of 0.5 to 10% by weight in terms of yttrium oxide based on the total amount of the titania-zirconia-yttria.
(24) The process for producing a titania-zirconia powder as set forth in (21), wherein the powerful stirring in step (b) is carried out uniformly in a homogenizer.
(25) The process for producing a titania-zirconia powder as set forth in (21), wherein the drying and calcining in step (c) is carried out at a rate of temperature rise of 100xc2x0 C./hr or lower.
(26) The process for producing a titania-zirconia powder as set forth in (21), wherein the raw material aqueous solution further comprises an aluminum salt.
(27) The process for producing a titania-zirconia powder as set forth in (21), further comprising the step of preparing a raw material aqueous solution comprising an aluminum salt, wherein a pH adjusting agent is added to each of the raw material aqueous solution comprising the titanium salt and the zirconium salt and the raw material aqueous solution comprising the aluminum salt, and the formed precipitated is uniformly mixed.
The present invention provides a titania-zirconia powder, a yttria-containing titania-zirconia powder, and a titania-zirconia-alumina powder which exhibit excellent resistance to poisoning by acidic substances and excellent properties of desorbing the acidic substances, maintains a large specific surface area even after a high-temperature treatment, and can have its heat resistance markedly increased without changing the characteristic of titania that there are few basic sites on the surface. Therefore, the present invention provides powders which are suitable as a catalyst carrier or a co-catalyst, particularly as a catalyst carrier or a co-catalyst for purifying automotive exhaust gases or as a catalyst carrier for use in high temperature gases containing sulfur.